European Patent No. 0 124 842 describes a PFBC power plant in which, for control of the power, hot bed material is transferred from the bed vessel to a storage container upon a power reduction and is returned to the bed vessel upon a subsequent power increase. Swedish patent application No. 8403162-4 describes a PFBC power plant with a storage container for a hot bed material which is utilized for ignition of the combustion chamber. In such storage containers, slag lumps may be formed which are of such a size that they may cause clogging of the return conduit and thus prevent the return of the bed material to the combustion chamber. The object of the invention is to design the plant in such a way as to eliminate the above-mentioned problems arising in connection with the return of bed material from the storage container.